1. Field of the Invention
This invention relates to transmission measurements in carrier telephone circuits, and more particularly to remote loop transmission testing between terminals of a carrier system.
2. Description of the Prior Art
Techniques for the remote measurement of transmission loss, in both directions of transmission through a two-way circuit, have been given considerable attention for a long period of time. For example, in U.S. Pat. No. 2,666,099, issued Jan. 12, 1954, such a testing technique is disclosed. A disadvantage of this technique is that a separate 1000-cycle generator is required at each terminal. A further disadvantage is that some means for adjustment of a variolosser is required at the receiving terminal. Also, external transmission equipment is employed.
One technique for overcoming the requirement for external transmission equipment at one of the two locations is described in U.S. Pat. No. 3,526,729, issued Sept. 1, 1970, entitled "Transmission Measuring System With Harmonic Generating Means". Disadvantages of this technique are the use of a diode across the transmission path or other means for generating harmonics of the test signal, and the use of separate test equipment consisting of an oscillator for generating the test signal and a test meter for measuring the harmonic signal power. An improvement on the technique disclosed in U.S. Pat. No. 3,526,729 is given in U.S. Pat. No. 3,660,620, issued May 2, 1972, entitled "Transmission Measurement With A Two-Component Signal". Disadvantages of this technique are the use of a diode network which is connected across the transmission path and the need for two different oscillators (frequency generators), one of which acts as a carrier frequency.
Another technique is disclosed in U.S. Pat. No. 3,739,107, issued June 12, 1973, entitled "On Premise Telephone Loop Tester". In particular, this reference requires a separate loop tester which in effect closes the tip-and-ring circuit of a telephone line to simulate an "off-hook" condition. Further, the loop tester must be deactivated during normal use of the telephone in order to prevent actuation by voice energy. The predetermined audio tone transmitted from the central office to the subscriber station is also external to the telephone transmission equipment.